Photochromism is the reversible function of a certain compound that it changes its color swiftly upon exposure to light including ultraviolet light such as sunlight or light from a mercury lamp and returns to its original color when it is put in the dark by stopping its exposure to light. A compound having this property is called “photochromic compound” and used as a material for photochromic plastic lenses.
For the photochromic compound used for this purpose, the following properties are required: (I) the degree of coloration at a visible light range before ultraviolet light is applied (to be referred to as “initial coloration” hereinafter) should be low, (II) the degree of coloration upon exposure to ultraviolet light (to be referred to as “color optical density” hereinafter) should be high, (III) the speed from the time when the application of ultraviolet light is started to the time when the color optical density reaches saturation (to be referred to as “color development sensitivity” hereinafter) should be high; (IV) the speed from the stoppage of the application of ultraviolet light to the time when the compound returns to its original state (to be referred to as “fading speed” hereinafter) should be high, (V) the repeat durability of this reversible function should be high, and (VI) the solubility in a monomer composition which will become a host material after curing of the photochromic compound should be high so that its dispersibility in the host material in use becomes high.
As the photochromic compound which can satisfy these requirements, there are known chromene compounds having an indeno(2,1-f)naphtho(1,2-b)pyran structure as the basic skeleton (refer to a pamphlet of International Laid-Open WO99/15518 and a pamphlet of International Laid-Open WO2001/60811).
It is preferred that a photochromic plastic lens comprising the photochromic compound should develop a color of a neutral tint such as gray or brown. A color of a neutral tint is obtained by mixing together several different kinds of photochromic compounds which develop different colors, for example, a yellow to red photochromic compound (yellow compound) having a maximum absorption at 430 to 530 nm and a purple to blue photochromic compound (blue compound) having a maximum absorption at 550 to 650 nm.
However, when color control is carried out by this method, various problems occur due to the difference in photochromic properties between the compounds which have been mixed together. For example, when the repeat durability of the yellow compound is lower than that of the blue compound and the photochromic plastic lens is used for a long time, there occurs a problem that the developed color gradually changes to a color of a strong blue tint.
Further, when the color development sensitivity and fading speed of the yellow compound are lower than those of the blue compound, there arises a problem that color during development has a strong blue tint and color during fading has a strong yellow tint.
It is considered that this problem can be solved by using a single compound which has two or more absorption maximums at the time of exposure and develops a color of a neutral tint (double peak compound). It is known that the yellow compound is generally inferior to the blue compound in durability. Therefore, a compound having higher yellow color optical density (having a maximum absorption wavelength at 430 to 530 nm) than blue color optical density (having a maximum absorption wavelength at 550 to 650 nm) is desired as the double peak compound (the ratio of the yellow color optical density to the blue color optical density in the double peak compound may be referred to as “double peak characteristic” hereinafter). When the ratio of the yellow color optical density to the blue color optical density is 0.8 or more to less than 1.1, a good gray color is obtained and when the ratio is 1.1 to 2.0, a good brown color is obtained. Therefore, when the double peak characteristic is less than 0.8, a blue or purple tint becomes strong and when the double peak characteristic is 2.0 or more, a yellow or orange tint becomes strong.
As the photochromic compound having two or more absorption maximums at the time of color development (double peak compound), there are known compounds represented by the following formulas (A) to (G) and having an indeno(2,1-f)naphtho(1,2-b)pyran structure as the basic skeleton.
However, these compounds have room for the improvement of the following points. That is, a chromene compound represented by the following formula (A) (refer to a pamphlet of International Laid-Open WO01/19813) has low color development sensitivity, low fading speed and low repeat durability though its double peak characteristic is high.

A chromene compound represented by the following formula (B) (refer to a pamphlet of International Laid-Open WO05/028465) has strong initial coloration as the end of its absorption spectrum (to be referred to as “absorption end” hereinafter) goes beyond 420 nm into the visible range though it has excellent double peak characteristic and practical levels of color optical density and fading speed. Although the absorption end is existent at a long wavelength range, as will be described hereinafter, it has low color development sensitivity. Therefore, this chromene compound has room for the improvement of this point as well.

A chromene compound represented by the following formula (C) (refer to a pamphlet of International Laid-Open WO08/054,942) has low color optical density though it has a practical level of fading speed.

A chromene compound represented by the following formula (D) (refer to a pamphlet of International Laid-Open WO06/110221) has low color optical density though it has a practical level of fading speed.

A chromene compound represented by the following formula (E) (refer to a pamphlet of International Laid-Open WO05/028465) has low double peak characteristic with a smaller absorption at 430 to 530 nm than an absorption at 550 to 650 nm as well as low durability.

A chromene compound represented by the following formula (F) which has a trifluoromethylsulfonyloxy group at the 7-position is disclosed by US2009/0309076A1. However, this compound has low double peak characteristic with a smaller absorption at 430 to 530 nm than an absorption at 550 to 650 nm.

Further, a compound having a specific substituent at the 11-position is disclosed by a pamphlet of International Laid-Open WO05/090327). Stated more specifically, a chromene compound represented by the following formula (G) is disclosed.

However, this compound has low double peak characteristic with a smaller absorption at 430 to 530 nm than an absorption at 550 to 650 nm. Also, since its fading speed is very low, it has room for the improvement of this point as well.
Further, in recent years, a photochromic compound which develops a color even inside a car has been desired, and the further improvement of color development sensitivity has been desired.